Fluidized bed incinerators are known for burning and decomposing refuse such as municipal wastes. In the method of incineration/decomposition in a fluidized bed incinerator, refuse in burned while being fluidized in a fluidized bed. In order to improve combustion of the refuse in the fluidized bed, a fluidizing medium such as sand is fed together with the refuse to form the fluidized bed.
The typical type of fluidized bed incinerator is equipped with a plurality of air diffuser tubes or plates (called as "air diffusers" hereinafter) supplying air to the lower part of the incinerator body, and with a refuse feeding mechanism and a fluidizing medium feeding mechanism in the upper part thereof.
The refuse in burned while both the refuse and the fluidizing medium are deposited onto the air diffuser tubes or plates inside the incinerator body, where they are fluidized by primary air blown from the air diffusers.
The refuse, which may comprise municipal wastes, generally contains a variety of materials such as low calorie refuse such as food, high calorie refuse such as plastics and rubber, refuse such as shredded paper or chipped furniture, and refuse such as fragmented metallic or vitreous containers, bottles, or cans.
As the refuse is fed to the fluidized bed, the combustibles are burned, of which substances such as plastics undergo pyrolysis and generate various pyrolysis gases, while the incombustibles such as metals or glass remain unburned (called "combustion residue" hereinafter).
As the fluidizing medium is gradually fed to the fluidized bed, a moving bed of fluidizing medium is formed which descends as the fluidizing medium is continuously supplied. As the combustibles are burned and decomposed within the fluidized bed, the combustion residue flows out of the incinerator, together with the fluidizing medium, through the gaps among the air diffusers at the lower section of the fluidized bed. The fluidizing medium and the combustion residue are separated from each other, and the separated fluidizing medium is returned to the fluidized bed.
Secondary air is supplied to the upper section of the incinerator, where the generated pyrolysis gases are burned.
Because, in this type of fluidized bed incinerator, the sand which is used as the fluidizing medium and which is deposited onto the fluidized bed is oscillated while it descends and is heated, the agitation and dispersion of the refuse is promoted.
For this reason, the refuse deposited onto the fluidized bed is dispersed uniformly in the presence of the fluidizing medium, and is dried, ignited, burned and decomposed instantaneously. Further, the ashes and dust produced therein rise to upper section the incinerator and are collected by an electric precipitator.
Consequently, the refuse deposited onto the fluidized bed is disposed of almost completely, leaving behind some metallic, vitreous, or ceramic residue. The ratio of these substances to the refuse is usually 2%, meaning a fluidized bed incinerator can dispose of 98% of the refuse. A primary advantage of the fluidized bed incinerator is that the volume of combustion residue can be reduced to 1/3 compared to a conventional mechanical incinerator, such as a stoker-type combustor.
A problem exists, however, with fluidized bed incinerators in that the refuse deposited onto the fluidized bed is burned and decomposed at a speed so high that the refuse cannot be stably combusted. The refuse has different caloric value depending on the particular content of the refuse, and it is often difficult to supply a constant volume of the refuse to the fluidized bed. For example, suppose that a large quantity of refuse is deposited onto the fluidized bed at once, that the refuse is burned and decomposed instantaneously and that a large amount of pyrolysis gas along with dust is then generated, simultaneously. In this instance, it is not only impossible to completely combust the resulting large amount of pyrolysis gas with secondary air inside incinerator but it is also difficult to collect all of the large amount of smut in the exhaust gas using the electric precipitator.
The principal object of this invention is to provide a method and apparatus for slowing the burning and decomposition of the refuse in the fluidized bed to achieve stable combustion.
Further, another object is to control the speed of the fluidized bed and to be able to carry out stable combustion despite the fluctuations in the volume of refuse deposited onto the fluidized bed.
An additional object of this invention is the provision of a method and apparatus for stable combustion for a fluidized bed incinerator which is capable of reducing the volume of air supplied for the combustion of the refuse and of maintaining the combustion temperature of pyrolysis at a high level in the combustion chamber.